Edit Mode Crosshair Cursor Invisible in 4K resolution
This is the culprit:

wm_cursors.c:
#ifdef _WIN32
/* the default win32 cross cursor is barely visible,
* only 1 pixel thick, use another one instead */
if (curs == CURSOR_EDIT)
curs = BC_CROSSCURSOR;
#else


I downloaded the source code and compiled blender myself to remove this. This way it uses windows's default precision cursor which you can change to whatever you want.